A spindle apparatus includes a base, such as a housing, and a spindle supported so as to be capable of rotation with respect to the base. A tool is fitted to the spindle, either directly or via a tool holder. A spindle apparatus having electrical contact between the base and the spindle in order to supply electrical current from the base side to the spindle and lead sensor signals from the spindle to the base side is known. Sensor signals may be signals indicating contact between a tool and a workpiece. Japanese patent No. 3238218 and Japanese examined patent application No. 3-058851 disclose a spindle apparatus in which electrical current for electric discharge machining is supplied to a tool via the spindle. In Japanese patent No. 3238218, a conducting ring is provided on the spindle, and a brush is pressed against the conducting ring in a radial direction. In patent application No. 3-058851, a brush is pressed against the spindle in a radial direction. It is often the case that the brush is brought into contact with the spindle in this way in order to supply electrical current from the base side to the rotating spindle.
In recent years, there has been advancement in the improvement of spindle bearings, etc. in response to calls for improved machining precision, etc. of machine tools. As a result, spindle apparatus have been proposed that are capable of rotating a spindle about its axis with remarkably high precision. With a spindle apparatus in which a brush is pressed against the spindle, rotating the spindle about its axis with high precision is difficult. In order to reliably connect the base and the spindle, it is necessary to increase the pressing force of the brush on the spindle. However, a higher pressing force increases axial vibration of the spindle. On the other hand, a weak pressing force causes insufficient contact of the brush with the spindle, and as a result electrical contact between the base side and the spindle becomes unstable.